This invention relates to a method for reducing the cyanide ion and heavy metal content of aqueous solutions of plating wastes. More particularly, it relates to an improvement in the known method of electrolytically reducing the concentration of these contaminants employing a novel dual electrode, usable as a cathode or an anode, which facilitates metal recovery from the solutions and is capable of reducing the metal content of the effluent stream to low levels in a single pass.
Plating wastes comprise aqueous solutions of silver, copper, cadmium, chromium, or other heavy metals. The anion associated with the metal is usually cyanide or sulfate. Conventional plating waste treatment methods have developed slowly and with an emphasis upon the removal of a particular component from a particular waste stream. Currently, the most common method for removing inorganic heavy metals from solutions prior to their disposal is by chemical precipitation. However, different metals precipitate under various conditions, e.g., at various pH levels. When two or more heavy metals are found in the same waste stream, the optimum conditions for precipitation will often be different for the different ions.
Another prior art method for treating waste water is by a cementation process wherein scrap iron, steel wool, or zinc dust is used to cement copper, silver, gold, cadmium, mercury or lead. Both of these conventional treatments substitute new ions for those removed.
Recently, attempts have been made to purify such waste waters by electrochemical methods. Examples of these techniques are disclosed in Destruction of Cyanide Wastes Within a Packed Bed Electrode, Chin et al (Plating And Surface Finishing, Vol. 63, October, 1976); Electrochemical Removal of Silver Ions From Photographic Fixing Solutions Using a Porous Flow Through Cathode, Van Zee et al (Electrochemical Science And Technology, Vol. 124, May, 1977); and U.S. Pat. No. 3,970,531, H. L. Recht (July 20, 1976). In these and similar processes, the dilute solutions to be purified are passed through a cell having porous, flow-through high surface area electrodes. Typically, the waste solution is recirculated through the cell until the effluent reaches a selected low level. Because of the high surface area of the cathode and the relatively long residence time, large percentages of the ion contents of the waste solutions are plated out at the cell cathode, and cyanide is destroyed by oxidation at the anode. However, the costs of operating this type of purification system has retarded extensive commercial exploitation.
Waste water treatment by conventional processing cannot normally be justified on economic grounds except in limited cases such as those involving the recovery of silver from photographic fixer solutions. Treatment of such waste waters could be improved if the process flow sheet were simplified, the volume of solid waste (which typically require treatment such as dewatering and sludge disposal) was reduced, or the treatment resulted in the production of a solid waste from which valuable metals could be recovered at a reasonable cost.